Natural Radionuclides ²²²Rn and ²²⁶Ra in Shallow Groundwater of Nysa County (SW Poland): Concentrations, Background, and Radiological Risk

Abstract

Natural radionuclides may occur in groundwater and pose health risks when present in elevated concentrations. This study evaluates the quality of shallow groundwater in Nysa County (SW Poland) based on the activity concentrations of natural radionuclides radon (²²²Rn) and radium (²²⁶Ra) and estimates the associated radiological risk from water ingestion. Twenty-three groundwater samples were collected from private wells located within two distinct geological units: the Fore-Sudetic Block and the Opole Trough. Activity concentrations of ²²²Rn and ²²⁶Ra were measured using the liquid scintillation counting method. A spatial distribution model for ²²²Rn was developed using inverse distance weighting in QGIS. Local hydrogeochemical background levels were determined using the Q-Dixon test, interquartile range, and Shapiro–Wilk normality test. The background ranged from 2.6 to 3.9 Bq·L⁻¹ in the Opole Trough and from 0 to 10.7 Bq·L⁻¹ in the Fore-Sudetic Block. The lower detection limit (0.05 Bq·L⁻¹) for ²²⁶Ra activity concentration measurements was not exceeded. Effective dose rates were calculated in accordance with the recommendations of the International Commission on Radiological Protection and United Nations Scientific Committee on the Effects of Atomic Radiation. Doses ranged from <1 µSv to over 120 µSv·y⁻¹. Although all samples met national regulatory standards (≤1 mSv·y⁻¹), the World Health Organization reference level (0.1 mSv·y⁻¹) was exceeded in two cases. The results support the need for the radiological monitoring of unregulated private wells and provide a scientific basis for the refinement of legal frameworks and health protection strategies.

1. Introduction

In Poland, the administrative structure includes counties (powiats), which serve as the second level of territorial division between municipalities and provinces. As of 1 January 2025, Poland had 314 counties and 66 cities with county status, covering a wide geographic and demographic range [1]. Nysa County, located in the southwestern part of the country, covers an area of 1224 km² and has a population of approximately 127,000 [1].

Geologically, the county lies at the junction of two large structural units: the Fore-Sudetic Block (part of the Lower Silesian Block) and the Opole Trough. The western and southwestern parts of the county lie within the Fore-Sudetic Block, while the eastern part lies within the Opole Trough (Figure 1). The Fore-Sudetic Block, first named by Stupnicka in 1989 [2], is a vast tectonic unit that is depressed relative to the Sudetes Mountains bordering it to the southwest. The Sudetes and the Fore-Sudetic Block, which form the Lower Silesian Block, were distinguished as a result of vertical tectonic movements occurring in the Neogene, accompanying the block-like uplift of the Bohemian Massif on the foreland of the forming Alpine orogen, in the final phase of the formation of the Alps and Carpathians [3,4].

The Fore-Sudetic Block in Poland borders the Sudetic Marginal Fault to the southwest and, in the Czech part, the East-Sudetic Flexure [5]. To the northeast, the Fore-Sudetic Block borders the Fore-Sudetic Monocline, with the boundary between these two units being defined by the Middle Oder Horst [4]. In terms of structural geology, the eastern boundary of the Fore-Sudetic Block should be assumed to correspond to the extent of the faults marking the western edges of the Permian–Triassic Basin of the Fore-Sudetic Monocline, covered by Upper Cretaceous sediments [3].

Figure 1. Geological regional subdivision of Fore-Carpathian Poland at the sub-Cenozoic palaeosurface [6] (modified): USCB—Upper Silesian Coal Basin; Nysa County is marked with a solid red line.

Figure 1. Geological regional subdivision of Fore-Carpathian Poland at the sub-Cenozoic palaeosurface [6] (modified): USCB—Upper Silesian Coal Basin; Nysa County is marked with a solid red line.

The youngest Quaternary sediments in the study area are primarily of postglacial origin and may include rock fragments with uranium minerals. In the southern part of the Fore-Sudetic Block, surface rocks, not found in the Opole Trough, include phyllites, granites, paragneisses, mica schists with amphibolite interbeds, quartzites and quartzite schists with actinolite quartzite interbeds, biotite gneisses, granites, granodiorites, and tonalites [7]. These lithologies may contain rocks potentially rich in natural radionuclides. The distribution of ²²⁶Ra in the soil of the Nysa County region is estimated at 20–40 Bq·kg⁻¹ [8].

Radon (²²²Rn) is a colorless, odorless, and tasteless noble gas. It is the longest-lived isotope of radon, with a half-life of about 3.82 days [9]. It is produced by the α-decay of the alkaline earth element ²²⁶Ra, the longest-lived isotope of radium, with a half-life of about 1600 years [10,11].

Consuming water containing radionuclides such as ²²⁶Ra and ²²²Rn is a significant source of internal exposure to ionizing radiation. Studies have shown that radium ions are human carcinogens and can increase the risk of cancer in various tissues and organs to differing degrees [12,13,14]. Furthermore, the ingestion of radium-contaminated water has been identified as a significant risk factor for the development of upper gastrointestinal cancers [15]. The cationic form of radium is chemically similar to calcium, which leads to its accumulation in bone tissue [16]. This, in turn, results in the irradiation of bone marrow and an increased risk of developing bone cancers (including sarcomas), as well as leukemia [17,18]. Ingested ²²²Rn enters the gastrointestinal tract, where its decay products emit radiation into the stomach wall, potentially increasing the risk of gastrointestinal malignancies. It may also be absorbed into the bloodstream and distributed throughout the body [19]. The consumption of such alpha-emitting radionuclides can also lead to DNA damage, thereby increasing the risk of cancer even in somatic cells located far from the initial site of accumulation [18,20].

According to legislation currently in force in Poland, the mandatory monitoring of ²²²Rn and ²²⁶Ra content applies to tap water supplied to households or water used for medicinal purposes [21,22]. Many studies conducted in Poland to date have primarily focused on regions where crystalline rocks (igneous and metamorphic) occur at shallow depths [11,23,24,25,26,27,28,29,30,31].

The hydrogeochemical background of ²²²Rn in groundwater for the Fore-Sudetic Block has been estimated at 2–47 Bq·L⁻¹ [32], while, for the Fore-Sudetic Monocline bordering it to the north, it is estimated at 1–7 or 0–11, depending on the methodology used [33]. The hydrochemical backgrounds of surface waters in units within the Lower Silesian Voivodeship adjacent to Nysa County have also been determined. It ranges from 0.26 to 1.33 Bq·L⁻¹ for the Sudetes, 0.18 to 0.92 Bq·L⁻¹ for the Fore-Sudetic Block, and 0.12 to 0.48 Bq·L⁻¹ for the Fore-Sudetic Monocline [34]. However, so far, there have been no published data on the content of radionuclides in shallow circulating groundwater in the entirety of Nysa County.

The aim of this study was to assess the potential occurrence of elevated ²²²Rn and ²²⁶Ra activity concentrations in water from the first aquifer (underground domestic intakes), which is not subject to mandatory monitoring but is commonly used for domestic purposes. This highlights the need for regular monitoring of the radiological quality of these waters. They are particularly susceptible to enrichment with the radioactive isotope ²²²Rn, as the activity concentration of ²²²Rn in shallow waters is higher than in deep waters [35]. This can be attributed to rock weathering processes, which enhance the release of radon and its precursors (radium and uranium), occurring primarily in the near-surface layers of the lithosphere. These processes lead to mineral disintegration and an increase in the radon emanation coefficient. Since the extent of weathering rarely exceeds 100–200 m, the highest radon concentrations are typically observed in shallow groundwater. This relationship has been confirmed, among others, in the Sudetes, in both crystalline and sedimentary rocks [36,37].

Furthermore, infiltrating rainwater rapidly flows through the aeration zone, flushing radon from intergranular spaces. As it percolates deeper, its concentration gradually decreases due to radioactive decay and the limited availability of radium, particularly in rocks with low porosity and few fractures. The decreasing porosity and intensity of weathering with depth further reduce radon emanation. This has also been observed in other regions, such as Western Switzerland, where a significant correlation was found between the yield of karst springs and the radon concentration in the discharged groundwater [11,38,39].

However, in some areas, the radon concentrations in groundwater have been observed to increase with depth. This phenomenon has been attributed to the larger volume of reservoir rocks and the greater surface area for water–rock interaction at depth. It may occur particularly in systems with rapid groundwater flow, such as karst aquifers, or in areas where the uranium content varies significantly across geological layers [38,39,40,41].

Moreover, this study involved determining radionuclide concentrations and establishing the local hydrogeochemical background for the parts of the Fore-Sudetic Block and the Opole Trough located in the area of Nysa County.

Additionally, the authors estimated the effective dose of ionizing radiation that could be received by users of groundwater in the study area. The permissible annual effective dose for the general population is 1 mSv. The obtained results were compared with applicable national standards [22] and the World Health Organization (WHO) recommendations [42], and their interpretation was related to the geological conditions of the region and aspects of radiological protection.

2. Materials and Methods

2.1. Determination of the Activity Concentrations of ²²²Rn and ²²⁶Ra

The liquid scintillation method was used to determine the activity concentration of ²²²Rn in groundwater. Scintillation vessels composed of potassium-depleted glass were prepared at the Laboratory of Earth Sciences and Mineral Engineering, Wrocław University of Science and Technology, and 10 cm³ of Insta-Fluor™ Plus (Revvity) liquid scintillator was poured into each vessel. The vessels were then weighed to the nearest thousandth of a gram. Next, a water sample, collected as part of the field study, of approximately 10 cm³, was injected into the prepared vessels. Two independent water samples were collected from each sampling site (Figure 2). Due to the screening nature of the study, water samples were collected from sources that met the following criteria: the water was actively used (ensuring flow and representativeness), and there was technical feasibility to collect the sample. Additionally, site selection aimed to ensure a relatively even spatial distribution across the study area, allowing for a preliminary assessment of spatial variability in the analyzed parameters. The date and time of sampling were carefully recorded, and the water sample was immediately injected under the liquid scintillator layer using a sterile needle and syringe. The vessel containing the scintillator and water sample was shaken vigorously to ensure that the ²²²Rn dissolved in the water transferred to the scintillator layer. The samples collected in this way were transported to the laboratory, where they were weighed again (to determine the mass of the collected water) and placed in an ultra-low-level α/β Quantulus 1220 liquid scintillation spectrometer (PerkinElmer, now Revvity), characterized by a very low background level.

The spectrometer is equipped with a scintillation detector. Its operation is based on the scintillation phenomenon and photoelectricity effect. By using differences in pulse duration and pulse shape analysis (PSA), it is possible to distinguish alpha and beta particles and record their spectra. The choice of scintillation cocktail (Insta-Fluor™ Plus) has a significant impact on both the counting efficiency and the ability of the pulse shape analyzer to discriminate between α and β pulses. Different cocktails vary in their scintillation decay times, which directly affects the accuracy in assigning recorded pulses to the appropriate energy windows. For Insta-Fluor™ Plus, the PSA threshold was determined experimentally using a reference source emitting both α and β radiation. This approach minimized spectral overlap and ensured the correct separation of α signals, which is particularly important given that some radon progeny, such as ²¹⁸Po and ²¹⁴Po, are α emitters and could otherwise interfere with data interpretation.

The lower limit of detection (LLD) was established following the calibration procedure of the Quantulus 1220 spectrometer. Both standard solutions of known activity concentrations and background samples were analyzed to determine the instrument’s sensitivity. Under the applied measurement conditions, the LLD for ²²²Rn was found to be 0.05 Bq·L⁻¹, a value consistent with the manufacturer’s specifications and confirming the suitability of the method for the reliable determination of ultra-low radon concentrations in water. Achieving the lowest possible LLD was crucial for the research due to the lack of prior data on possible ²²²Rn activity concentrations in the study area, especially within the Opole Trough.

The water samples remained in the ultra-low-level α/β Quantulus 1220 liquid scintillation spectrometer for approximately five hours. This was necessary to achieve a radioactive equilibrium between the ²²²Rn isotope and its short-lived daughters and to extinguish the flashes resulting from white light exposure. Additionally, this period of time allowed the samples to cool and ensured that the ²²²Rn activity concentration was measured at a constant temperature of 18.2 °C.

The spectrometer was programmed using the WinQ software (version 1.3) to measure each of two water samples in nine cycles, lasting 60 min each. This resulted in the acquisition of 18 results regarding the ²²²Rn activity concentration from a single sampling point. Subsequently, the authors performed a statistical analysis and rejected outliers. The reported ²²²Rn activity concentration is a weighted average of the measurements from two samples with equal measurement uncertainty.

The concentration of ²²²Rn activity in water at the moment of its outflow from the aquifer was determined using Formula (1) [44]:

$$\mathrm{c} = {\displaystyle \frac{{\mathrm{\lambda }}_2({\mathrm{k}}_{2\mathrm{\alpha }\mathrm{a}}\sum {\mathrm{L}}_{2\mathrm{\alpha }\mathrm{a}}^{\ast }-{\mathrm{L}}_{\mathrm{P}\mathrm{o}})}{{\mathrm{V}}_{\mathrm{p}}{\mathrm{k}}_{\mathrm{t}2\mathrm{\alpha }\mathrm{a}}{\mathrm{e}}^{-{\mathrm{\lambda }}_2{\mathrm{t}}_{0\mathrm{a}}}(1-{\mathrm{e}}^{-{\mathrm{\lambda }}_2{\mathrm{t}}_{\mathrm{a}}})}}$$

(1)

where

• c—²²²Rn activity concentration in water [Bq·dm⁻³];
• λ₂—²²²Rn decay constant [s⁻¹];
• k_2αa—correction factor [–];
• ∑L*_2αa—number of recorded ²²²Rn pulses and its derivatives in time from t_0a to t_0a + t_a [–];
• L_Po—number of ²¹⁰Po decays in time from t_0a to t_0a + t_a [–];
• V_p—volume of water sample tested [dm³];
• k_t2αa—emission factor ²²²Rn [–]; if the measurement is performed within 5 h to 2 days after taking the water sample, it can be assumed = 3.01;
• t_0a—time counted from the moment of water flowing out of the aquifer until the start of the measurement in the spectrometer (until the beginning of the pulse counting process) [s];
• t_a—measurement duration (pulse count duration) [s].

The ²²²Rn measurements were non-destructive, allowing the same vessels to be used for future measurements of ²²⁶Ra content. Based on the known ²²²Rn activity concentration, the time required for the complete decay of the originally dissolved ²²²Rn (to below the LLD) was calculated. This ensures that, after remeasuring the ²²²Rn activity concentration, the detected activity originates exclusively from the decay of ²²⁶Ra isotopes present in the water. After the incubation, the ²²²Rn activity concentration was measured again, and the obtained values were equivalent to the ²²⁶Ra activity concentration in the tested water sample. This method thus allowed for the reliable determination of the ²²⁶Ra concentration without additional sampling or chemical procedures.

To develop a model of the spatial distribution of ²²²Rn activity concentrations in groundwater in Nysa County, inverse distance weighting (IDW) interpolation was used in QGIS. This method, commonly used in environmental analyses, calculates a value for each cell based on the values of nearby points, weighted by the inverse of their distance. As a result, the further a given point is from the location being interpolated, the less influence it has on the calculated value [45,46,47].

2.2. Determination of the Hydrogeochemical Background

The hydrogeochemical background is defined as the range of values of hydrogeochemical properties or concentrations for substances tested that are characteristic of a given unit, its subregion, or the environment. This background is limited by upper and lower concentration thresholds. Values outside these ranges are considered anomalous [48]. Based on the ²²²Rn activity concentration data, the authors determined the local hydrogeochemical background level for ²²²Rn.

To establish the hydrogeochemical background levels of ²²²Rn separately for the Fore-Sudetic Block and the Opole Trough, a multi-stage statistical analysis was conducted. The first step involved identifying gross errors. Because the group size was below 25, the Dixon Q test was used. After arranging the data in an ascending sequence, the Q coefficient was determined, which is the quotient of the difference between the result closest to the questionable result and the questionable result, and the difference between the smallest and largest results. The calculated Q values were compared with the critical Qt value for a significance level of α = 0.05. Outliers and extreme values were then identified using the interquartile range (IQR). Outliers were defined as values located more than 1.5 × IQR from the upper (Q1) or lower (Q3) quartile, while extreme values were those exceeding 3 × IQR from Q1 or Q3 [49,50,51]. After rejecting gross errors, outliers, and extreme values, the normality of the data distribution was tested using the Shapiro–Wilk W test. If the distribution was normal, the hydrogeochemical background was calculated as the range Z ± 1.96σ, where Z is the arithmetic mean and σ is the standard deviation [52,53].

2.3. Estimation of the Effective Dose of Ionizing Radiation

The effective doses of ionizing radiation that a user of the tested water may receive from its consumption were estimated in accordance with the recommendations of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR; [54]), as well as those presented by National Research Council [55] and the International Commission on Radiological Protection (ICRP; [56]). Based on the available dose conversion factors, the UNSCEAR guidelines allowed the estimation of doses for three age groups—infants, children (1, 5, 10, and 15 years), and adults—whereas the ICRP provided dose estimates for adults only. Annual water consumption was assumed to be 150 L for infants, 350 L for children, and 500 L for adults [57]. The dose conversion factors for each group are summarized in

Table 1. Effective dose conversion factors [55,56].

Source	Age Group [−]	Effective Dose Conversion Factor [Sv·Bq−1]
National Research Council [55]	Adults	3.5 × 10−9
	Infants	4 × 10−8
	1 year old	2.3 × 10−8
	5 years old	1 × 10−8
	10 years old	5.9 × 10−9
	15 years old	4.2 × 10−9
ICRP [56]	Adults	6.9 × 10−10

.

The annual effective dose (Sv·year⁻¹) was determined as the product of the ²²²Rn activity concentration (Bq·L⁻¹), the effective dose conversion factor (Sv·Bq⁻¹; Table 1), and the annual volume of consumed water (L⁻¹·year⁻¹), according to Formula (2) [54]:

AED_ig = C_Rn × EDC × C_W

(2)

where

• AED_ig—annual effective dose received through ingestion [Sv·year⁻¹];
• C_Rn—²²²Rn activity concentration [Bq·L⁻¹];
• EDC—effective dose conversion factor [Sv·Bq⁻¹];
• C_W—annual volume of water consumed [L·year⁻¹].

3. Results and Discussion

3.1. Activity Concentrations of ²²²Rn and ²²⁶Ra

All groundwater samples showed concentrations of ²²²Rn above the lower detection limit for the applied liquid scintillation method, which was 0.05 Bq·L⁻¹. In contrast, the ²²⁶Ra concentrations in all cases remained below this limit, suggesting the trace presence of this radionuclide in the waters of the studied region (

Table 2. Results of measurements of ²²²Rn and ²²⁶Ra activity concentrations within the Nysa district.

No.	Date of Sampling	Location	222Rn [Bq·L−1]	226Ra [Bq·L−1]
1	22 March 2025	Jarnołtówek	10.13 ± 0.91	<0.05
2		Podlesie	7.19 ± 0.71	<0.05
3		Stary Las	2.60 ± 0.36	<0.05
4		Wilamowice Nyskie	20.63 ± 1.51	<0.05
5		Biała Nyska	2.96 ± 0.39	<0.05
6		Domaszkowice	5.12 ± 0.56	<0.05
7		Rynarcice	3.75 ± 0.46	<0.05
8		Stara Jamka	1.49 ± 0.25	<0.05
9		Rzymkowice	2.80 ± 0.38	<0.05
10		Niesiebędowice	8.75 ± 0.84	<0.05
11		Giełczyce	3.37 ± 0.36	<0.05
12		Prusinowice	3.13 ± 0.35	<0.05
13	13 April 2025	Chróścina	1.38 ± 0.22	<0.05
14		Rzymiany	2.48 ± 0.32	<0.05
15		Jędrzychów	4.17 ± 0.45	<0.05
16		Łąka	16.33 ± 1.20	<0.05
17		Jasienica Góra	6.11 ± 0.60	<0.05
18		Wilamowa	0.95 ± 0.18	<0.05
19		Kamienica	6.10 ± 0.62	<0.05
20		Maciejowice	0.23 ± 0.08	<0.05
21		Otmuchów	5.92 ± 0.60	<0.05
22		Goworowice	9.92 ± 0.86	<0.05
23		Szklary	5.03 ± 0.54	<0.05

).

Two important hydrological factors influencing the variability in radionuclide concentrations in groundwater are the aquifer recharge dynamics and water residence time. The rapid infiltration of meteoric or surface waters can shorten the contact time between water and host rocks, affecting the degree of radon emanation and transport to the phreatic zone. For example, studies have employed ²²²Rn as a tracer to identify zones of surface water with aquifer mixing and infiltration-driven recharge and have shown that radon concentration gradients can reflect spatial recharge patterns [58,59].

In systems characterized by slower recharge or longer residence times, radon may decay or degas before transport, potentially leading to lower measured activity concentrations in groundwater samples [60].

The ²²²Rn activity concentration values in the tested samples ranged from 0.23 ± 0.08 Bq·L⁻¹ (well 20; Maciejowice) to 20.63 ± 1.51 Bq·L⁻¹ (well 4; Wilamowice Nyskie). The highest ²²²Rn activity concentrations were recorded in water from wells located in Wilamowice Nyskie (20.63 ± 1.51 Bq·L⁻¹), Łąka (16.33 ± 1.20 Bq·L⁻¹), Jarnołtówek (10.13 ± 0.91 Bq·L⁻¹), and Goworowice (9.92 ± 0.86 Bq·L⁻¹).

The spatial distribution of the ²²²Rn activity concentrations is shown in Figure 3.

Increased ²²²Rn activity concentrations in water samples may be associated with the occurrence of crystalline basement outcrops, especially in the southern part of the study area. In the case of three elevated values, one each in the west, northwest, and east of the district, they may be related to the presence of sandstone formations with increased contact with basement rocks. The presence of fault zones, well documented in this area, may also favor the transport of ²²²Rn from deeper levels of the Earth’s crust to the saturation zone, increasing its content in shallow groundwater circulation [7,61,62].

The lower ²²²Rn activity concentrations observed, for example, in Wilamowa (0.95 ± 0.18 Bq·dm⁻³) and Maciejowice (0.23 ± 0.08 Bq·dm⁻³) may be related to the local geological structure (e.g., dominance of clay sediments; loess and loess-like clays in Wilamowa and boulder clays in Maciejowice), but also to anthropogenic operational factors—for example, the occasional or irregular use of the intake, which may lead to water stagnation and a decrease in the ²²²Rn concentration due to radioactive decay before sampling. Although clay minerals, especially 1:2-type phyllosilicates, are known to enhance the retention of ²²⁶Ra and consequently support the higher emanation of ²²²Rn into groundwater [63], the measured activity concentrations in the studied wells remained low, typically a few Bq·L⁻¹. This may be explained by the limited ²²⁶Ra content in the local loess and boulder clay sediments and the low permeability, which can restrict radon transport, as well as potential degassing due to intermittent well use. Therefore, while the process of Ra retention and radon emanation is scientifically valid, its contribution under the local hydrogeological and operational conditions appears minor. During the study, efforts were made to ensure that water was collected from wells in regular use.

As for the radium isotope ²²⁶Ra, none of the samples showed a concentration exceeding the LLD, confirming its low mobility under the hydrodynamic conditions prevailing in the first aquifer of the studied formations. This also indicates the absence of increased ion exchange activity between the water and the Ra-containing rock matrix. The contrasting behavior of ²²⁶Ra and ²²²Rn results from their geochemistry. Radium, as an alkaline earth metal, has limited solubility and often remains fixed in mineral lattices or adsorbed on clay surfaces, while radon, as a noble gas, readily diffuses from rock grains into pore water. In partially saturated aquifers, radon is continuously released from ²²⁶Ra-bearing minerals into the water phase, even though the radium itself does not migrate significantly [64].

The radioactive decay of ²²²Rn can reduce activity in long-residence waters if the original radon supply is limited. Therefore, depending on the local groundwater flow conditions, stagnation may cause an increase or decrease in measured activity compared to rapidly renewed systems. Although radon is soluble in water, its residence time in the aqueous phase is limited, as it continuously diffuses through the water–air interface. The “shielding effect” of water molecules is therefore only temporary. Degassing is strongly enhanced by turbulence, aeration, pressure drops during pumping, and temperature increases. These processes explain why even low-flow domestic wells may show reduced radon activity at the point of sampling compared to the stagnant groundwater within the aquifer. The phenomenon is well documented and has been exploited in practical mitigation: aeration treatments can remove up to 99.9% of dissolved radon [55,65]. Field studies in private wells further demonstrate that household use, especially showering, causes rapid radon transfer to indoor air, with short-term indoor concentrations reaching several thousand Bq·m⁻³ [66]. This confirms that groundwater acts only as a transient reservoir of radon, rather than a long-term protective medium.

3.2. Hydrogeochemical Background

The hydrogeochemical background for ²²²Rn was determined separately for samples from areas of the district parts the Fore-Sudetic Block and the Opole Trough. The first step involved analysis using the Dixon Q test. The critical values (Qt) at a significance level of α = 0.05 were 0.554 and 0.521 for the Opole Trough and the Fore-Sudetic Block datasets, respectively [67]. The calculated Q values for the minimum and maximum observations were 0.015 and 0.492 for the Opole Trough and 0.035 and 0.211 for the Fore-Sudetic Block. These values were therefore lower than the Qt values. This means that no gross errors occurred at the assumed significance level.

The interquartile ranges (IQRs) were 1.620 and 5.775 for the Opole Trough and the Fore-Sudetic Block, respectively. Based on these, outliers and extreme values were identified. Outliers were defined as those that were more than 1.5 × IQR from Q1 or Q3, while extreme values were defined as those that were more than 3 × IQR from Q1 or Q3. For the Opole Trough and the Fore-Sudetic Block, three and one outlier values and one extreme value, respectively, were detected. The Shapiro–Wilk W test confirmed the normal distribution of the data (p values for the Fore-Sudetic Block and the Opole Trough were 0.6779 and 0.9958, respectively).

The range of the hydrogeochemical background for ²²²Rn was determined to be 2.6 ÷ 3.9 Bq·L⁻¹ for the Opole Trough and 0 ÷ 10.7 Bq·L⁻¹ for the Fore-Sudetic Block. All measured ²²²Rn activity concentration values for both units, which were not classified as outliers or extremes, fall within these ranges. These values are consistent with previous studies conducted in adjacent geological units [32,33]. Although outliers and extreme values were excluded from the background calculations, their significance should be emphasized. They may indicate local tectonic or hydrogeochemical anomalies that deserve further detailed investigation.

None of the samples exceeded the national reference threshold for ²²²Rn in drinking water (100 Bq·L⁻¹; [22]), which confirms that the analyzed intakes do not pose a threat from a formal and legal perspective. However, as further analysis of effective doses shows, legal standards do not always align with international recommendations.

3.3. Effective Doses

Based on the measured ²²²Rn activity concentrations, effective doses associated with water consumption for different age groups were estimated (

Table 3. Annual effective doses received through ingestion as a result of the annual consumption of a specific volume of water from the studied locations, estimated using two methods.

No.	ICRP [μSv·y−1]	UNSCEAR [μSv·y−1]
	Adults	Adults	Infants	1 Year Old	5 Years Old	10 Years Old	15 Years Old
1	3.50 ± 0.30	17.70 ± 1.60	60.75 ± 5.45	34.95 ± 3.15	15.20 ± 1.40	9.00 ± 0.80	6.40 ± 0.60
2	2.45 ± 0.25	12.55 ± 1.25	43.15 ± 4.25	24.85 ± 2.45	10.80 ± 1.10	6.35 ± 0.65	4.55 ± 0.45
3	0.90 ± 0.10	4.55 ± 0.65	15.60 ± 2.20	8.95 ± 1.25	3.90 ± 0.50	2.30 ± 0.30	1.65 ± 0.25
4	7.10 ± 0.50	36.10 ± 2.60	123.75 ± 9.05	71.20 ± 5.20	30.95 ± 2.25	18.25 ± 1.35	12.95 ± 0.95
5	1.05 ± 0.15	5.20 ± 0.70	17.75 ± 2.35	10.25 ± 1.35	4.45 ± 0.55	2.65 ± 0.35	1.85 ± 0.25
6	1.80 ± 0.20	8.95 ± 0.95	30.75 ± 3.35	17.65 ± 1.95	7.65 ± 0.85	4.50 ± 0.50	3.25 ± 0.35
7	1.30 ± 0.20	6.60 ± 0.80	22.50 ± 2.80	12.95 ± 1.55	5.60 ± 0.70	3.30 ± 0.40	2.40 ± 0.30
8	0.50 ± 0.10	2.60 ± 0.40	8.90 ± 1.50	5.15 ± 0.85	2.25 ± 0.35	1.30 ± 0.20	0.95 ± 0.15
9	0.95 ± 0.15	4.90 ± 0.70	16.80 ± 2.30	9.65 ± 1.35	4.20 ± 0.60	2.45 ± 0.35	1.75 ± 0.25
10	3.00 ± 0.30	15.30 ± 1.50	52.50 ± 5.00	30.20 ± 2.90	13.15 ± 1.25	7.75 ± 0.75	5.50 ± 0.50
11	1.15 ± 0.15	5.90 ± 0.60	20.25 ± 2.15	11.65 ± 1.25	5.05 ± 0.55	3.00 ± 0.30	2.10 ± 0.20
12	1.10 ± 0.10	5.50 ± 0.60	18.80 ± 2.10	10.80 ± 1.20	4.70 ± 0.50	2.80 ± 0.30	2.00 ± 0.20
13	0.50 ± 0.10	2.40 ± 0.40	8.30 ± 1.30	4.75 ± 0.75	2.05 ± 0.35	1.20 ± 0.20	0.85 ± 0.15
14	0.85 ± 0.15	4.35 ± 0.55	14.90 ± 1.90	8.60 ± 1.10	3.70 ± 0.50	2.20 ± 0.30	1.60 ± 0.20
15	1.45 ± 0.15	7.30 ± 0.80	25.00 ± 2.70	14.35 ± 1.55	6.25 ± 0.65	3.70 ± 0.40	2.60 ± 0.30
16	5.60 ± 0.40	28.60 ± 2.10	98.00 ± 7.20	56.35 ± 4.15	24.50 ± 1.80	14.45 ± 1.05	10.25 ± 0.75
17	2.10 ± 0.20	10.65 ± 1.05	36.70 ± 3.60	21.05 ± 2.05	9.20 ± 0.90	5.40 ± 0.50	3.85 ± 0.35
18	0.35 ± 0.05	1.65 ± 0.35	5.70 ± 1.10	3.30 ± 0.60	1.45 ± 0.25	0.85 ± 0.15	0.60 ± 0.10
19	2.10 ± 0.20	10.70 ± 1.10	36.60 ± 3.70	21.05 ± 2.15	9.15 ± 0.95	5.35 ± 0.55	3.85 ± 0.35
20	0.10 ± 0.01	0.40 ± 0.10	1.40 ± 0.50	0.80 ± 0.30	0.35 ± 0.15	0.20 ± 0.10	0.15 ± 0.05
21	2.00 ± 0.20	10.35 ± 1.05	35.50 ± 3.60	20.45 ± 2.05	8.90 ± 0.90	5.25 ± 0.55	3.75 ± 0.35
22	3.40 ± 0.30	17.40 ± 1.50	59.55 ± 5.15	34.25 ± 2.95	14.90 ± 1.30	8.75 ± 0.75	6.25 ± 0.55
23	1.70 ± 0.20	8.80 ± 0.90	30.15 ± 3.25	17.35 ± 1.85	7.55 ± 0.85	4.45 ± 0.45	3.15 ± 0.35

). The table shows the annual effective dose of ionizing radiation that could be received due to consuming a specific volume of water with a given ²²²Rn activity concentration each year. The effective dose conversion factors recommended by the ICRP and UNSCEAR were used (Table 1).

The effective doses for adults ranged from 0.10 ± 0.01 to 7.10 ± 0.50 μSv·year⁻¹ using the ICRP conversion factors and from 0.40 ± 0.10 to 36.10 ± 2.60 μSv·year⁻¹ using the UNSCEAR conversion factors. A comparison of the results showed that the UNSCEAR conversion factors led to significantly higher effective dose values than those adopted by the ICRP.

The highest effective dose values were calculated for water samples from wells 4 (Wilamowice Nyskie) and 16 (Łąka). For infants, who are particularly sensitive to radiation due to their low body weight, the maximum estimated effective doses, in accordance with the pessimism principle, were 132.8 μSv·year⁻¹ and 105.2 μSv·year⁻¹ for water from wells 4 and 16, respectively. These exceed the WHO reference level for drinking water, which is 0.1 mSv·year⁻¹ [42]. This means that, in these two cases, the water does not meet the WHO recommendations, despite meeting national regulations for water intended for human consumption [22]. The dose received by an infant through water consumption may exceed even 13% of the annual dose limit. According to the WHO recommendations [42], if the effective dose is equal to or lower than the reference level (0.1 mSv·year⁻¹), the water is considered safe for human consumption, and no additional action is required. However, if the dose exceeds this level, remedial actions should be undertaken to reduce it.

The cancer risk resulting from water consumption was estimated using a nominal risk coefficient of 5.5 × 10⁻²·Sv⁻¹ [68]. In this study, the maximum annual risk was approximately 7.3 × 10⁻⁶ (i.e., 0.073‰), which should be considered low. Nonetheless, given that the analyzed water intakes are unregulated, the periodic monitoring of their quality is recommended.

4. Conclusions

In Nysa County, 23 groundwater samples were analyzed, collected from domestic wells located within two geological units: the Opole Trough and the Fore-Sudetic Block. These studies allowed for the assessment of natural radionuclide concentrations (²²²Rn and ²²⁶Ra) and the associated radiological exposure for potential users of these waters.

In none of the water samples did the ²²⁶Ra content exceed the lower detection limit of 0.05 Bq·L⁻¹, indicating the absence of significant sources of this radionuclide in the aquifer reservoir rocks.

The ²²²Rn activity concentrations in the samples ranged from 0.23 to 20.63 Bq·L⁻¹. The highest values were recorded in the south of the study area, most likely related to the presence of local granite, granodiorite, and tonalite outcrops, as well as numerous fault zones.

The determined range of the ²²²Rn hydrogeochemical background was estimated at 2.6 ÷ 3.9 Bq·L⁻¹ for the Opole Trough and 0 ÷ 10.7 Bq·L⁻¹ for the Fore-Sudetic Block. These results are consistent with previously determined background values for adjacent units. Outliers, including the highest recorded value of 20.63 ± 1.51 Bq·L⁻¹, may indicate local geochemical or tectonic anomalies, which may become the subject of future research.

The effective doses of ionizing radiation resulting from water consumption ranged from <1 µSv·year⁻¹ to more than 120 µSv·year⁻¹, depending on the location and age group. The maximum dose for an infant (132.8 µSv·year⁻¹) exceeds the WHO reference level of 0.1 mSv·year⁻¹ for drinking water [42].

The tested waters meet the standards specified in national regulations [22], considering the parameter values for ²²²Rn (100 Bq·L⁻¹) and ²²⁶Ra (0.05 Bq·L⁻¹). Their consumption does not exceed the annual ionizing radiation dose limit of 1 mSv for the general population [69]. However, in accordance with the WHO recommendations [42], if the effective dose exceeds 0.1 mSv·year⁻¹, drinking water should not be used for consumption without remedial measures. Therefore, in at least two locations, water from domestic wells does not meet international guidelines, despite compliance with national regulations. This highlights the need for the development and implementation of unified legal regulations.

The estimated radiation-induced cancer risk from consuming the tested water is very low (maximum 7.3 × 10⁻⁶·year⁻¹ for infants, i.e., 0.073‰), but it should not be disregarded.

Despite the low ²²²Rn potential in the Nysa district, the authors emphasize the necessity of monitoring all groundwater intakes used for human consumption, rather than only those connected to public water supply systems.